The present invention relates to digitizing tablets and their manner of operation and, more particularly, to an improved digitizer employing induced voltages and having reduced interference caused by adjacent AC fields comprising, a frequency source capable of producing a plurality of frequencies suitable for use as a transduction frequency; a movable instrument including an electrical element of an electrical conductor; a tablet having a digitizing surface with one or more conductors disposed across the digitizing surface; means for determining the position of the movable instrument on the digitizing surface from voltages induced between the conductors and the electrical element at a transduction frequency; and, means for selecting the one of the frequencies to be employed as the transduction frequency wherein, with particularity, the means for selecting the one of the frequencies to be employed as the transduction frequency includes means for testing voltages induced between the conductors for flutter and for employing as the transduction frequency the one of the frequencies which minimizes any flutter.
Digitizing tablets (large and small) are popular support items for use with graphics-oriented systems such as CAD/CAM, and the like. Particularly in commercial grade operations, they afford a quick and accurate way to input positional information on a two-dimensional X-Y oriented surface. As shown in FIGS. 1 and 2, a typical digitizer 10 comprises a grid of equally spaced vertical X-axis wires 12 and horizontal Y-axis wire 14. The wires 12, 14 are generally spaced about 0.2-0.4 inches apart. the X-axis wires 12 are connected to an X-axis wire selector 16, while the Y-axis wires 14 are connected to a Y-axis wire selector 18. The wire selectors 16, 18 cause the wires 12, 14, respectively, to be selected in a time sequence as indicated by the dashed arrows 20, 22. One mode of operation for the digitizer 10 is depicted in simplified form in FIGS. 2 and 3. The X-axis wires 12 and Y-axis wires 14 are perpendicular to one another on alternate, non-touching layers. The reading of each layer is accomplished separately and comprises sensing the voltage induced by the oscillating magnetic field of the transducer 13 in each of the wires 12, 14. The transducer 13 (whether in the form of a "puck" or stylus) includes a coil of wire 24 having a pair of leads 26 therefrom on which a signal can be imposed by oscillator 30. As the coil of wire 24 within the transducer 13 is moved over the surface 28 of the digitizer 10, its position is detected by sensing the magnitudes and phases of the voltages on the wires 12, 14 buried within the digitizer 10 below the surface 28. With reference to FIG. 2 in particular, at scan time "t", current flowing in the coil 24 causes current flow in the wire 12 in one direction while in the wire 12 at can time "t+1" the induced current in the wire 12 causes current flow in the opposite direction. The change in current direction is indicative of which two wires 12, 14 the coil 24 is disposed between. In actual practice, the phase and magnitude of the induced wire currents are interpolated to determine the location at which a wire would have zero induced current and thus the location of the center of the transducer 13; but, such interpolation techniques are not important to the present invention and, therefore, will not be described in detail herein. In an alternate approach which is also not vital to the present invention, the wires 12, 14 are sequentially energized and the induced signal in the coil 24 is sensed to detect the coil (and associated transducer) position.
The problem addressed by the present invention is depicted with reference to FIGS. 4-5. Regardless of the approach employed, the coupling of AC magnetic fields between the coil 24 and the wires 12, 14 is the mechanism by which the position of the the coil 24 is determined. As shown in FIG. 4, it is typical to position the digitizer 10 close to the CRT display monitor 36 for ease of use; that is, an operator sits looking at the monitor 36 while employing the digitizer 10 to make positional inputs in real time. The potential for problem exists as shown in FIG. 5 and is a function of the monitor type, raster scan rate thereof, position relative to the digitizer, etc. As is known, the visual display on the monitor 36 is created by the continual sweeping from left to right and top to bottom of an electron beam across a phosphor surface on the inside of the CRT 36. What may not be generally realized and appreciated, however, is that the magnetic field from the beam deflection yoke in the monitor extends beyond the phosphor surface as indicated by the field lines 44. The flux of this field through the loop created by wires 12, 14 and their ground return to the tablet amplifier of the digitizer induces a small voltage therein. The effect of these magnetic fields may be understood with reference to FIG. 6. As depicted therein, the demodulator alternately applies gains of + 1 and then -1 to the amplified grid signal, producing a signal with a DC level, which is positive if the demodulator input is in phase with the transduction frequency signal and negative if it is out of phase. This particular non-linear process of demodulation, like all non-linear processes where two signals are combined, produces new frequencies corresponding to the sum and difference of the two input frequencies.
The amplified wire voltage signal is effectively being multiplied by a square wave with a range of +1 to -1. Although this effective square wave contains frequency components of all odd multiples of its fundamental frequency, consideration of only the effects of this fundamental frequency is sufficient to illustrate the problem created by the monitor fields 44. Thus, a signal on the amplified wire voltage other than that coupled from the transducer 13 will, when combined in the demodulator 38 with the transduction frequency, produce frequencies at the demodulator output equal to the sum and difference of the extraneous signal and the transduction frequency. In the case of the adjacent monitor, the field produced by the horizontal sweep is similar to that depicted in FIG. 7a while the voltage induced into the grid wires 12, 14 by this field is the time derivative as depicted in FIG. 7b. This waveform can be shown to contain frequency components at higher multiples of its fundamental frequency. Thus, a 15.75 KHz monitor produces harmonics of 31.5, 47.25, 63.0, 78.75 KHz, and higher frequencies. A digitizer operating as indicated in FIG. 6 with a transduction frequency of 80 KHz would produce frequencies of 158.75 KHz and 1.25 KHz at the demodulator output in response to the 78.75 KHz harmonic. The lowpass filter 39 would effectively eliminate the 158.75 KHz component; but, would be ineffective at eliminating the 1.25 KHz component. Due to the large number of different brands and types of monitors and terminals, it is difficult to find a transduction frequency that is far enough from all the different monitor frequencies and harmonics to ensure stable digitizer operation.
As mentioned earlier, the wires 12, 14 are spaced apart and the precise position of the coil 24 is determined by interpolation between the two wires on opposite sides of it. Where resolution of 200-400 lines per inch is desired, the voltages induced into the wires 12, 14 by the deflection yoke fields 44 are insignificant compared to the changes in voltage per line of resolution. Where resolution in the neighborhood of 1000 per inch is desired, however, the induced voltages from the fields 44 can be significant, causing the signal being interposed to flutter, producing corresponding flutter in the computed position of the transducer 13.
It should be noted in passing that the use of multiple or various frequencies for transducer purposes, per se, is known in the part. Ikedo (U.S. Pat. No. 4,088,842) describes the use of multiple frequencies in the use of multiple transducers on the same digitizer. Both Ikedo and Rogers (U.S. Pat. No. 4,672,154) describe the use of multiple frequencies for communicating transducer switch activations to the digitizing tablet.
Wherefore, it is the principle object of the present invention to provide a general method and associated apparatus for the operation of a high resolution digitizer which eliminates or minimizes flutter caused by induced voltages from an adjacent display monitor, or the like.
Other objects and benefits of the present invention will become apparent from the description which follows when taken in conjunction with the drawing figures which accompany it.